To find a good calling card or evaluate a calling card, you should know how those operators are running their business.
Normally they lease transmission lines from AT&T or other carriers, and then they put their traffic through. At the same time, they have several standby choices. Maybe tomorrow, MCI will give them a lower rate, then they will switch the traffic from AT&T to MCI. They can never guarantee the quality. You may have a good speech quality today, and you find it is difficult to call tomorrow with the same company.
It is almost impossible to evaluate the quality of service.
My advice: the card should not be expensive, and should not be too cheap either.